trapme: If using a TC custom target, ensure the CUSTOM_TARGET field is valid

[sxa]

From July TCK retrospective issue, there have been issues with people manually making a mess of the CUSTOM_TARGET field. It would be useful for the harness to check for two specific situations which have been hit by multiple team members (at least myself and @steelhead31):

• If the CUSTOM_TARGET field has a prefix of CUSTOM_TARGET= which it should have
• If the CUSTOM_TARGET field is missing the testSuite= at the end of the field.